At the heart of a computer is a magnetic disk drive that includes a magnetic disk, a slider where a magnetic head assembly including write and read heads is mounted, a suspension arm, and an actuator arm. When the magnetic disk rotates, air adjacent to the disk surface moves with it. This allows the slider to fly on an extremely thin cushion of air, generally referred to as an air bearing. When the slider flies on the air bearing, the actuator arm swings the suspension arm to place the magnetic head assembly over selected circular tracks on the rotating magnetic disk, where signal fields are written and read by the write and read heads, respectively. The write and read heads are connected to processing circuitry that operates according to a computer program to implement write and read functions.
Typically magnetic disk drives have been longitudinal magnetic recording systems, wherein magnetic data is recorded as magnetic transitions formed longitudinally on a disk surface. The surface of the disk is magnetized in a direction along a track of data and then switched to the opposite direction, both directions being parallel with the surface of the disk and parallel with the direction of the data track.
Data density requirements are fast approaching the physical limits, however. For example, increased data capacity requires decreased bit sizes, which in turn requires decreasing the grain size of the magnetic medium. As this grain size shrinks, the magnetic field required to write a bit of data increases proportionally. The ability to produce a magnetic field strong enough to write a bit of data using conventional longitudinal write head technologies is reaching its physical limit.
One means for overcoming this physical limit has been to introduce perpendicular recording. In a perpendicular recording system, bits of data are recorded magnetically perpendicular to the plane of the surface of the disk. The magnetic disk may have a relatively high coercivity material at its surface and a relatively low coercivity material just beneath the surface. A write pole having a small cross section and very high flux emits a strong, concentrated magnetic field perpendicular to the surface of the disk. This magnetic field emitted from the write pole is sufficiently strong to overcome the high coercivity of the surface material and magnetize it in a direction perpendicular to its surface. This flux then flows through the relatively magnetically soft underlayer and returns to the surface of the disk at a location adjacent a return pole of the write element. The return pole of the write element has a cross section that is much larger than that of the write pole so that the flux through the disk at the location of the return pole (as well as the resulting magnetic field between the disk and return pole) is sufficiently spread out to render the flux too week to overcome the coercivity of the disk surface material. In this way, the magnetization imparted by the write pole is not erased by the return pole.
Efforts to minimize track width and bit size when using perpendicular recording have focused on the formation of the write pole since the write pole defines both the track width and the bit size. Most desirably, the write pole should have a trapezoidal, or tapered shape in order to prevent adjacent track writing problems associated with skew. As those skilled in the art will recognize, skew occurs as an actuator arm swings the magnetic head to either extreme of its pivot range (ie. at the inner and outer portions of the disk). Such skew positions the head at an angle, which positions portions of the write pole outside of the desired track. Forming the write pole with a trapezoidal shape reduces such adjacent track writing.
Another attempt to improve write pole performance has focused on reducing remnance. Remnance is the slower than desired magnetization decay when the write current is turned off. Because a large amount of flux is being forced into a relatively small write pole, when the write current is turned off the magnetization of the write pole does not immediately cease, but continues for an undesirably long period of time. An approach to alleviate this has been to form the write pole as laminations of magnetic layers having very thin layers of non-magnetic material disposed therebetween.
Efforts to form the desired trapezoidal, laminated write poles have involved forming laminated layers of high Bsat magnetic material and then depositing a hard mask and a photoresist patterning mask. A material removal process such as reactive ion etch (RIE) has then been used, with the photoresist as a mask, to pattern the hard mask. Ion milling has then been used to remove the magnetic material there under. An angled ion milling process has then been used to form the desired tapered shape of the write pole.
A problem that has been encountered with the above, however, is that due to poor RIE selectivity between the hard mask and the photoresist mask layer, the photoresist mask layer must be made very thick. This is because a large amount of the photoresist must be consumed in the patterning of the hard mask. As increased data densities require smaller track widths, the tall photoresist structure becomes problematic. For example it would be desirable to use deep ultraviolet (deep U.V.) photolithography or e-beam photolithography, because these processes provide high resolution and allow a well defined small track width write pole to be constructed. However, in general, thicker resist degrades resolution due to worsening aerial imaging in the case of deep UV lithography, and increased blurring due to forward scattering in the case of e-beam lithography. In addition, since the aspect ratio (height to width) of a photoresist mask is limited by physical capabilities of the material, as track widths decrease the thickness must likewise decrease.
Therefore, there is a need for a process for forming a write pole of a perpendicular write head wherein the photoresist mask thickness (height) can be reduced while still achieving desired patterning of an underlying hard mask structure. Such a process would preferably allow the use of deep U.V. or e-beam photolithography and would allow the write head to be formed with a very narrow track width.